Method of forming composite articles using a mandrel assembly

ABSTRACT

A mandrel assembly has a hollow main tube and two annular sleeves, wherein an inner diameter of the two annular sleeves is larger than an outer diameter of the two main tube ends. The two annular sleeves are sleeved respectively on the two main tube ends, which can be easily disengaged from the mandrel assembly when applied with an external force.

This is a divisional of application Ser. No. 08/381,038 filed on Jan.31, 1995, now U.S. Pat. No. 5,580,416.

FIELD OF THE INVENTION

This invention relates to a mandrel assembly, especially to a mandrelassembly for use in a filament winding method.

BACKGROUND OF THE INVENTION

A filament winding method is to equally align the fiber bundle, wet thefiber bundle in a resin bath, regularly wind it in an assigned pattern,thickness and weight on a rotating mandrel, and cure and then strip thewound fiber bundle off the mandrel or strip off and then cure the woundfiber bundle on the mandrel.

Early products, e.g., a rocket engine cowling, a pressure vessel, abuoy, a conical tip of a missile, a storage tank, a cylindrical tube, amotor vane spring, a helicopter propeller, a spaceship spar, an airplanefuselage and other aerospace or industrial component parts, were mademainly in a cylindrical outline. After the fiber wetted with resin iswound on the mandrel in an assigned pattern, one has to cure the resincarried on the fiber with heat and then draw off the no longer requiredmandrel. So, the mandrel should be made of a material that is soluble orhas a low melting point (but still higher than the curing temperature ofthe resin) in order to pre-form the outline and size of a product, e.g.,a golf club. The filament winding process for making a carbon fiber golfclub is to wind a fiber that has been wetted in a resin bath in anassigned pattern on a conical iron core (i.e., a mandrel) mounted in awinding machine, and then wrap a heat shrinkable tape layer on the woundfiber such that the fiber wetted with the resin can be tightly formed onthe mandrel and the fiber can be cured in an oven. After stripping offthe tape layer and the mandrel, the wound fiber is ground to a requiredouter diameter in a centerless grinding machine to obtain a prototype ofthe golf club. However, we cannot manufacture sporting products such asbicycle handle bars, bicycle front forks, tennis rackets or badmintonrackets that have complex outlines, varying cross-sectional sizes orunsymmetrical straight cylindrical tubes by using the shrinkable tapeand applying the same curing procedure as used for a golf club. Instead,one has to use a mandrel that has an inflatable bladder inside byblowing up the mandrel to pressurize, harden and form a product.Consequently, it is necessary to remove the mandrel before the woundfiber can be cured.

A conventional filament winding method is to wind a fiber wetted withresin in an assigned pattern on an extractable mandrel, cure the woundfiber and then extract the mandrel. This, however, is not suitable forthe aforementioned product having a complex outline. Generally, for aproduct having a special requirement, the fiber is wound in an assignedpattern on a mandrel which is then heated until the resin carried on thefiber does not contaminate the operator's hand (i.e., a B-stage ofresin). The wound fiber is then stripped off the mandrel and put in anoven to be cured and formed. The design of a mandrel for products havingcomplex outlines, varying cross-sectional sizes and unsymmetricalstraight cylindrical tubes is therefore important. Conventionalextractable mandrel designs, e.g., illustrated in U.S. Pat. Nos.4,448,624, 4,684,423 and 5,259,901, are different from the presentinvention in both the configuration and the usage. Moreover, the mandrelused in a prior art manufacturing process, as shown in FIG. 1, has auniformly cross-sectional iron core 10 encapsulated with two or threelong arc strip shells 11. After sleeving an inflatable bladder on such amandrel, one may wind a fiber wetted with resin in an assigned patternon the mandrel that is mounted in a winding machine. Thereafter, thewound fiber, the arc strip shells 11 and the iron core 10 are put intoan oven to have the resin carried by the fiber cured to a B-stage. Theiron core 10 is then removed by a rapping device to leave a long hollowcylindrical configuration having inner arc shells 11, an inflatablebladder and an outer wound fiber. The arc shells 11 is then extracted toleave a final cylindrical strip with the wound fiber encapsulated aroundthe inflatable bladder. After putting the final cylindrical strip intoan open mold and closing the mold, one may blow air into the finalcylindrical strip to pressurize, heat and cure the product. Even thoughthe iron core 10, the arc shells 11 and the inflatable bladder can bedesigned specifically according to the size or outer diameter of aproduct, the design of the mandrel is very important due to the factthat it is very difficult to strip off a mandrel which is tightly woundby fiber with a high tension. The prior art manufacturing process whichuses an extractable mandrel can only strip off a mandrel by a rappingdevice and has the following defects:

1. It can only form a uniformly cross-sectional configuration;

2. It requires the use of a large number of iron cores and arc shellsfor mass production; and

3. It requires a rapping device which is costly.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a mandrel assemblywhich has an outline that can be adapted to the requirement of aproduct.

Another object of the present invention is to provide a mandrel assemblywhich can be easily disengaged from a fiber wound on the mandrelassembly.

Another object of the present invention is to provide a mandrel assemblywhich is suitable for mass production.

In accordance with the present invention, a mandrel assembly includes amain tube having two ends each having an outer diameter, and two annularsleeves respectively sleeved on the two ends of the main tube. Theannular sleeves having an inner diameter larger than the outer diameterof the two ends of the main tube so as to disengage from the main tubewhen applied with an external force. Each of the sleeves that has ataper ranged from 0.50 to 1.00 degree is fumed into a truncated conehaving a truncated end thereof facing outwardly when mounted on the maintube. The main tube is hollow and includes a middle hollow tube and twoend connecting tubes which have an outer diameter smaller than the outerdiameter of the middle hollow tube. The two annular sleeves are mountedon the two end connecting tubes, respectively. The middle hollow tubeand the end connecting tubes are integrally assembled, and the middlehollow tube includes two tube segments detachably sleeved together.

In accordance with another aspect of the present invention, a mandrelassembly further includes two end shafts respectively mounted on the twoends of the main tube such that the mandrel assembly can be mounted in awinding machine. Each of the end shafts includes a locking end forengaging a connecting piece of the winding machine and an opposite pinend mounted on a clip of the winding machine. A mandrel assembly furtherincludes an end shaft and a nozzle respectively detachably mounted onthe two ends of the main tube such that the mandrel assembly can bemounted on a winding machine.

In accordance with the present invention, the main tube is provided witha plurality of holes for discharging air blown into the main tubethrough the nozzle. The truncated end of each sleeve has a flangeadapted to be engaged by a tool so that each sleeve can be pulled outfrom the main tube upon releasing the mandrel assembly. Preferably, thesleeves are formed by winding a metal filament on a sleeve body which ismade of compressible material such as rubber, plastic, or paper. Thelength of the sleeves is in the range of from 80 mm to 150 mm. Apreferable length of the sleeves is about 100 mm. A taper of the annularsleeves is ranged from 0.50 to 1.00 degree, and a preferable taper ofthe sleeves is about 0.75 degree. The inner diameter of the annularsleeve is larger than the outer diameter of the main tube.

In accordance with another aspect of the present invention, the mandrelassembly further includes a shell surrounding the main tube in order toincrease an inner diameter of a product to be manufactured by themandrel assembly. The shell includes a plurality of arc pieces, e.g.,two hemi-cylindrical arc pieces.

In accordance with another aspect of the preset invention, the main tubeand the two annular sleeves are integrally formed. A main tube has twotube segments which are sleeved together and has a gradually outwardlyexpanding outer diameter. The main tube includes a middle hollow tubeand two end connecting tubes which have an outer diameter smaller thanthat of the hollow tube and mount respectively thereon the two annularsleeves. Further, the main tube includes two end shafts respectivelymounted on the two ends of the main tube in order that the mandrelassembly can be mounted in a winding machine.

In accordance with the present invention, a method for making a hollowproduct includes steps as follows:

a) providing a mandrel assembly including a main tube and two annularsleeves sleeved on the two ends of the main tube;

b) winding a fiber on the mandrel assembly;

c) disengaging at least one of the two annular sleeves from the maintube;

d) detaching the wound fiber from the main tube; and

e) shaping the wound fiber.

In accordance with the present invention, a method of making a hollowproduct further includes after the step a) a step of sleeving aninflatable bladder on the main tube and a step of wetting the fiber in aresin before winding the fiber on the mandrel assembly. The step e) isto heat the wound fiber until the resin carried by the fiber reaches aB-stage and to pressurize and heat the wound fiber in a closed mold inorder to produce the hollow product. Consequently, a method for makinghollow products includes steps of providing a main tube which has aplurality of holes and two end sleeves for mounting to two annularsleeves, mounting the main tube on a winding machine, winding a resinimpregnated fiber on the main tube, mounting an air nozzle at one of thetwo ends, blowing air through the nozzle to disengage at least one ofthe annular sleeves from the main tube, disengaging the wound fiber withthe main tube, curing the resin impregnated on the wound fiber, andshaping the wound fiber in a closed mold to form the follow product.

The present invention can best be understood through the followingdescription with reference to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic view showing conventional mandrel components;

FIGS. 2(A) and (B) are a schematic view and a top view respectivelyshowing a preferred embodiment of a mandrel assembly of the presentinvention;

FIG. 3 is a schematic view showing a fiber wound on a preferredembodiment of a mandrel assembly shown in FIGS. 2(A) and 2(B).

FIG. 4 is a schematic view showing a preferred embodiment of an annularsleeve and its auxiliary tool according to the present invention;

FIGS. 5(A) and 5(B) are a schematic view and a side view respectivelyshowing a preferred embodiment of the arc shells before and afterassembly according to the present invention;

FIG. 6 is a schematic view showing another preferred embodiment of theannular sleeve of the present invention;

FIG. 7 is a schematic view showing further preferred embodiment of anannular sleeve of the present invention;

FIG. 8 is a schematic view showing another preferred embodiment of themandrel assembly of the present invention;

FIG. 9 is a schematic view showing a further preferred embodiment of themandrel assembly of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 2(A), a preferred embodiment of a mandrel assemblyaccording to the present invention includes a hollow main tube 20 havingtwo middle hollow tubes 201 and 202 which respectively include aplurality of holes 207 and taperingly and mutually sleeve together toform an integral piece, two end connecting tubes 203 and 204 having anouter diameter smaller than that of the two middle hollow tubes 201 and202 respectively screwed to or taperingly sleeved on the two ends of themiddle hollow tubes 201 and 202, two annular sleeves 21 and 22 formedinto a truncated cone and preferably having a taper ranged from 0.5 to1.0 degree, a length determined according to the diameter and length ofthe product to be produced, and two flanges 211 and 221 respectivelyfixed on the outwardly faced ends thereof when respectively sleeved onthe two end connecting tubes 203 and 204 to respectively contact withthe two middle hollow tubes 201 and 202, wherein the outer diameter ofthe smaller end 212 (222) of each of the two sleeves 21 and 22 equals tothe outer diameter of the two ends 2011 and 2021 of the two middlehollow tubes 201 and 201, an end shaft 23 having a thread on its one endfor screwing to the end connecting tube 203, having the other endthereof mounted on a clip of a winding machine, and having a throughshaft hole communication with the end connecting tube 203 for passingtherethrough blown air, a locking end 24 having a thread on one endthereof for screwing to the end connecting tube 204 and an oppositve pinend to be mounted on a clip of a winding machine, a nozzle 25 having athread on one end thereof for screwing to the end connecting tube 203and the other end thereof connected to an air source, and at least a setof are shells 26, e.g., three cylindrical arc shells, surrounding atleast a portion of the middle hollow tube 201 and/or the middle hollowtube 202 so as to increase an outer diameter of the surrounding portion.

To assemble the mandrel of the preset invention is to sleeve togetherthe two middle hollow tubes 2O1 and 202, respectively, to mount the twoend connecting tubes 203 end 204 to the ends of the two middle hollowtubes 201 and 202 and to sleeve thereon the two annular sleeves 21 and22, to sleeve an inflatable bladder 27 on the middle hollow tubes 201and 202 when the product to be produced requires blown air in order tobe formed, to screw respectively the end shaft 23 end the locking end 24to the two end connecting tubes 203 and 204 as shown in FIG. 2(B), andto mount the mandrel assembly on a winding machine to wind the fiber ina pattern. The fiber (a carbon fiber or a glass fiber usually) is wettedin a resin before it is wound on the mandrel assembly. The two annularsleeves 21 and 22 are shaped like truncated cones because at the twoends of the main tube 20 where the winding procedure is backed, thefiber is usually more tightly wound, so as to allow the easy strip-offof the two annular sleeves 21 and 22. Further, the wound fiber has awider opening on each of its two ends to facilitate disengagement fromthe main tube 20. Referring to FIG. 3, to release the mandrel assembly,one replaces the end shaft 23 with a nozzle 15 for blowing airtherethrough into the two middle hollow tubes 201 end 202 having aplurality of holes 207 for detaching the inflatable bladder 27 and thewound fiber from the mandrel assembly and disengaging at least one ofthe two annular sleeves 21 and 22. If one of the two annular sleeves 21and 22 is not disengaged, one may pull off the annular sleeve by handwhen the tension of the wound fiber is small. When the tension is large,one may engage one of the sleeve flanges 211 and 221 by an auxiliarytool, e.g., a table brim or a fixing ring 31, to strip off the twoannular sleeves 21 and 22 as shown in FIG. 4. After the mandrel assemblyis disengaged from the winding machine and the annular sleeves 21 and 22are stripped off, the two end connecting tubes 203 and 204 are exposed.Since the end opening of the wound fiber is larger than the outerdiameter of the two connecting tubes 203 and 204 and further that, theinflatable bladder 27 and the wound fiber are detached from the mandrelassembly, the main tube 20 can be easily stripped off through one end ofthe wound fiber or through two ends of the wound fiber by separating themain tube 20 into two middle hollow tubes 201 and 202. One mayalternatively disengage one of the two connecting tubes 203 and 204, andtake off the corresponding annular sleeve 21 or 22 to expose theinflatable bladder 27, and blow air into the inflatable bladder 27 byclosing one of the two inflatable bladder ends to disengage the woundfiber together with the inflatable bladder 27 from the mandrel assembly.Finally, the wound fiber is placed in an oven until the resin carried onthe fiber reaches the B-stage, and then pressurized by blown air andheated in a closed mold in order to produce a hollow product.

In the preset invention, there are several preferred embodiments forreleasing a mandrel assembly in order to satisfy various processrequirements. For example, in the above-mentioned preferred embodimentfor winding a fibrous cylinder, one can stripp off the mandrel assemblyby first disengaging from the mandrel assembly the two annular sleeves21 and 22 without blowing air so that the plurality of holes 207 on thetwo middle hollow tubes 201 and 202 are not necessary. This is possiblewhen the tension of the wound fiber is very low. The two middle hollowtubes 201 and 202 may be integrally formed, and/or the two connectingtubes 203 and 204 may be integrally formed respectively with the twomiddle hollow tubes 201 and 202. One may also apply forces on the twoconnecting tubes 203 and 204 respectively to separate the two middlehollow tubes 201 and 202 with the assistance of blown air to reduce thefriction between the wound fiber and the mandrel assembly when thetension of the wound fiber is high. In this situation, the twoconnecting tubes 203 and 204 can still be integrally formed with the twomiddle hollow tubes 201 and 202.

Referring to FIGS. 5(A) and 5(B), where a mandrel assembly is used for aproduct that has a non-uniform cross section including a middle portionthat has a larger diameter, one may incorporate a set of arc shells 26on top of the two middle hollow tubes 201 and 202 to increase the outerdiameter of the middle portion and to wind a fiber on the inflatablebladder 27 sleeved over the mandrel assembly. The set of arc shellsincludes a plurality of arc sheels 26, e.g., either three arc shells oralternatively two hemi-cylindrical arc shells. To release the mandrelassembly, one may disengage the two sleeves 21 and 22 and the main tube20 by using the previously disclosed method, and then disengage thethree arc shells 26 by shaking. One may optionally decide the use of aplurality of holes 207 on the two middle hollow tubes 201 and 202. Thetwo middle hollow tubes 201 and 203 are integrally formed. The twoconnecting tubes 203 and 204 are integrally formed with the two middlehollow tubes 201 and 202, respectively according to the tension of thewound fiber on the mandrel assembly.

When the mandrel assembly is used for a product that has a non-uniformcross-section , i.e. a middle portion having a diameter smaller than thediameter of its two ends, the middle hollow tubes 201 and 202 are shapedto adapt to the varying diameter of the product. An inflatable bladder27 is then sleeved over the mandrel assembly. To release the mandrelassembly, one may disengage the two annular sleeves 21 and 22 by usingthe previously disclosed method, and then pull the two ends of theconnecting tubes 203 and 204 to separate and disengage the middle hollowtubes 201 and 202. One may optionally use a plurality of holes 207 onthe middle hollow tubes 201 and 202 The two connecting tubes 203 and 204are integrally formed with the two middle hollow tubes 201 and 202,respectively according to the tension of the wound fiber on the mandrelassembly.

Except for the disengagement of at least one of the two annular sleeves21 and 22 and the main tube 20, if the annular sleeves 11 and 22 aremade of a compressible material, i.e. a rubber or a plastic (as shown inFIG. 6) a papery material, or wound with a metal filament (as shown inFIG. 7), it is not necessary to form the annular sleeves 21 and 22 in atruncated cone since it is easy to disengage the annular sleeves 21 and22 from the middle hollow tubes 201 and 202 after the connecting tubes203 and 204 are first stripped off. The outer diameter of the annularsleeves 21 and 22 is larger than that of the middle hollow tubes 201 and202 when the annular sleeves 21 and 22 are sleeved on the connectingtubes 203 and 204, respectively such that they may be connected to themiddle hollow tubes 201 and 202 for easy disengagement from the maintube 20 due to the larger opening of the wound fiber. To release themandrel assembly, one may first disengage the connecting tubes 203 and204 because the annular sleeves 21 and 22 are elastic and then disengagethe two annular sleeves 21 and 22. For example, a plastic or a paperyAnnular sleeve can be slightly compressed and pulled out, while a woundfiber can be disengaged after the disengagement of a metal filamentwound annular sleeve. In this situation, the middle hollow tubes 201 and202 can be integrally formed.

Referring to FIG. 8, which shows another preferred embodiment that has anon-uniform cross-section (with a thinner middle portion). The mandrelassembly can be released as follows:

1. Designing a size for the middle portion in the middle hollow tube 80based on the product specification as shown in FIG. 8.

2. After winding, blowing air through a nozzle (at a pressure of about 2Kg/cm²) and holes 807 to slightly inflate the wound fiber. Pulling offthe annular sleeves 81 and/or 82 if they are not automatically ejectedby the blown air; and

3. Disengaging the annular sleeves 81 and 82 to expose the connectingtubes 803 and 804 out of the wound fiber and simultaneously, disengagingthe middle hollow tubes 801 and 802 from the ends of the hollow tube 80based on a smaller between the middle hollow tubes 801 and 802 and thewound fiber so as to complete the mandrel assembly release.

In the above preferred embodiment, the connecting tubes 803 and 804 canbe mounted on the middle hollow tubes 801 and 802, respectively withouta screw thread. When the tension of the wound fiber is very small, onemay apply the above mandrel release method to automatically eject (orpull out manually) the annular sleeves 81 and 82 without the need ofventilating holes 807.

FIG. 9 shows another preferred embodiment, wherein the middle hollowtubes 901 and 902 and the annular sleeves 91 and 92 are integrallyformed, respectively. Both of the middle hollow tubes 901 and 902 have agradually expanding outer end. The main tube 90 thus includes twomutually sleeved middle hollow tubes 901 and 902 that have a truncatedcomical end. One may disengage the main tube 90 in the wound fiber andpull it out in order to complete the mandrel release.

An example of a manufacturing process for a tennis racket is illustratedfor the present invention. After the number and thickness of the fiberlayers are determined based on the appearance, size and configuration ofthe product required (e.g., six fiber layers and a thickness of 1-2 mmfor each fiber layer), the size of each component of the mandrelassembly without an inflatable bladder can be calculated as follows:

Middle hollow tube: integrally formed to an outer diameter of 15.4 mmand to a total length of 1650 mm;

Hollow connecting tubes: integrally formed with the middle tube to anouter diameter of 13.4 mm and a length of 100 mm;

Truncated conical annular sleeve: formed to an inner diameter of 13.4mm, to two end outer diameters of 15.4 mm and 18 mm, respectively and toa length of 100 mm;

End shafts: formed with an M6 screw thread, to lengths of 20 mm and 40mm, respectively and to an outer diameter of 3.5 mm;

locking end: formed to 5/16 in.×18 pitch.

After the mandrel assembly is assembled and the inflatable bladder issleeved over the mandrel assembly, the tension of the inflatable bladderis adjusted to fit the annular sleeves on the connecting tubes such thatfiber can be wound in a predetermined pattern over the mandrel assembly.After the fiber is wound, air (at a pressure of about 2 Kg/cm²) is blownthrough a nozzle to disengage at least one of the two annular sleeves toexpose the two connecting tubes out of the wound fiber in order toremove the mandrel assembly. The annular sleeve is then pulled off tocomplete the removal of the mandrel assembly. Finally, the wound fibertogether with the inflatable bladder is heated to about 80° C. to curethe resin carried on the fiber to B-stage, and then the wound fiber isheated to about 150° C. and pressurize by blown air in a closed mold toproduce a tennis racket.

The range of the taper on the two annular sleeves 21 and 22 should bedetermined carefully. If the taper is too small, it would be difficultto release the mandrel assembly due to the large tension at the backingend of the wound fiber. If the taper is too large, the taper does notfacilitate the release of the mandrel assembly because the fiber slidesoff the back end easily. The inner diameter of the annular sleeve shouldtherefore match the outer diameter of the connecting tube at the hollowend and further, the diameter of the smaller end of the annular sleevemust equal to that of the middle hollow tubes 201 and 202. The taper ofthe two annular sleeves dependent upon the two end diameters and thelength thereof. Experiments show that the taper ranges from 0.50 to 1.00degree, and 0.75 degree is a preferable taper. The length can suitablyrange from 80 mm to 150 mm, with 100 mm as a preferable value. The fiberback end may suitably be located at the middle portion of each of theannular sleeves 21 and 22 in order to determine the length of each ofthe middle hollow tubes 201 and 202 based on the product produced.

The present invention has the following advantages:

1. The mandrel assembly can be easily changed based on a desiredproduct.

2. The mandrel assembly can be easily released;

3. The mandrel assembly can be used in mass production;

4. An inflatable bladder can be first installed before winding a fiberand then utilizing blown air, pressurization and heating for producingthe product;

5. A low cost method is provided.

6. The mandrel assembly has a light weight and can be operated easily.

Since the present invention mandrel assembly is easy to release, tostrip off or to disengage from a preformed wound fiber to allow theproduct to be placed in an oven to cure the resin to the B-Stage, themandrell assembly can be readily reused in the next production cycle.Furthermore, the components of the present invention mandrel assemblyare simple, lightweight and easy to manufacture. A total between fiveand ten mold assemblies would be sufficient for a mass productionprocess. This affords the present invention a &rest operating economyand a mass production capability when compared to the prior artindustrial techniques which require a much larger number of moldassemblies.

While the invention has been described in terms of what are presentlyconsidered to be the most practical and preferred embodiments, it is tobe understood that the invention need not be limited to thoseembodiments disclosed. On the contrary, it is intended to cover variousmodifications and arrangements within the spirit and scope of theappended claims which are to be accorded with the broadestinterpretation so as to encompass all such modifications and structures.

What is claimed is:
 1. A method for making a hollow product comprisingthe steps of:a) providing a mandrel assembly including a main tubehaving two ends and a plurality of holes and two annular sleeves sleevedon said two ends of said main tube; b) winding a fiber on said mandrelassembly; c) blowing air into said main tube for releasing said woundfiber from said mandrel assembly; d) disengaging at least one of saidannular sleeves from said main tube; e) detaching said wound fiber fromsaid main tube; and f) shaping said wound fiber.
 2. A method accordingto claim 1 further comprising a step of wetting said fiber in a resinbefore winding said fiber on said mandrel assembly.
 3. A method formaking a hollow product according to claim 2, wherein said step f) is toheat said wound fiber until said resin carried with said fiber reaches aB-stage and have said wound fiber pressurized and heated in a closedmold in order to produce said hollow product.
 4. A method for making ahollow product according to claim 1, wherein an inner diameter of saidannular sleeve is larger than an outer diameter of said main tube.
 5. Amethod for making a hollow product according to claim 1, wherein a taperof said annular sleeves is ranged from 0.50 to 1.00 degree.
 6. A methodfor making a hollow product according to claim 1, wherein a length ofsaid annular sleeves is ranged from 80 mm to 150 mm.
 7. A method formaking a hollow product according to claim 1, wherein said main tubeincludes a middle hollow tube and two end connecting tubes which have anouter diameter smaller than that of said hollow tube, said two annularsleeves are separable from said main tube and are mounted on said twoend connecting tubes respectively.
 8. A method for making a hollowproduct according to claim 1, wherein said main robe includes two endshafts respectively mounted on said two ends of said main tube in orderthat said mandrel assembly can be mounted on a winding machine.
 9. Amethod according to claim 1, further comprising after said stepa) a stepof sleeving an inflatable bladder on said main tube.
 10. A method formaking a hollow product comprising steps of providing a main tube havinga plurality of holes and two ends sleeving thereto two annular sleeves,mounting said main tube on a winding machine, winding a resinimpregnated fiber on said main tube, mounting an air nozzle on one ofsaid two ends, blowing through said nozzle to disengage at least one ofsaid annular sleeves from said main tube, disengaging said wound fiberwith said main tube, curing the resin impregnated fiber of said woundfiber, and shaping said wound fiber in a closed mold to form said hollowproduct.
 11. A method for making a hollow product comprising the stepsof:a) providing a mandrel assembly including a main tube having two endsand two annular sleeves sleeved on said two ends of said main tube; b)winding a fiber on said mandrel assembly; c) disengaging at least one ofsaid annular sleeves from said main tube; d) detaching said wound fiberfrom said main tube; and e) shaping said wound fiber, wherein said maintube including a middle hollow tube and two end connecting tubes whichhave an outer diameter smaller than that of said hollow tube and mountrespectively thereon said two annular sleeves.